Voice privacy device

ABSTRACT

A voice privacy coder system enables transmission of voice communication in privacy by first inverting the communication and then scrambling it directly with a complex code word. In one embodiment, a tone at the inversion frequency is also scrambled in with the voice communication and the complex code is added to the scrambled signal prior to transmission to provide synchronizing signals for the receiver; a decoder at the receiver reconstitutes the scrambled communication. In an alternate embodiment, the complex code is omitted from the transmitted signal and the inversion tone and code are reconstituted at the receiver by means of cross-coupled phase-lock and synchronizing loops.

United States Patent 1 Miller 1 Mar. 27, 1973 [54] VOICE PRIVACY DEVICEPrimary Examiner-Benjamin A. Borchelt [75] Inventor: Charles K. Miller,Concord, Mass. Assistant 'f Attorney-Cesar! and McKenna [73] Assignee:Technical Communications Corpora- -ti0[l, Lexington, M355. 1 filed:Julyao 1970 A voice privacy coder system enables transmission of [21] Apl. No. 59,659 voice communication in privacy by first inverting thecommunication and then scrambling it directly with a '[52] U S Cl 325/32179/1 5 R 179/1 5 S complex code word. In one embodiment, a tone at the[51] i n inversion frequency is also scrambled in with the voice 58Field o;;rc'i;IIIII32)'3 1s 2 i79 /i.5 s 1.5 R wmmunicafim and h isadded scrambled signal prior to transmission to provide 5 ReferencesCited synchronizing signals for the receiver; a decoder at the receiverreconstitutes the scrambled communication. UNITED STATES PATENTS ln analternate embodiment, the complex code is 2,479,338 8/1949 Gabrilovitch..179/1.s Omitted fwm the transmitted Signal and the inversion 2,905,7479/1959 'Kidd et al...... ...l78/5.l tone and code are reconstituted atthe receiver by 3,610,828 10/197l Girard et al. ...l79/l.5 means ofcross-coupled phase-lock and synchronizing 3,180,927 4/1965 Heppe et al....l79/l .5 l o 3,34l,659 9/l967 Stern ..l79/l.5

26 Claims, 2 Drawing Figures I021 6 2s L f s9 i r 1 t SHAPING BALANCEDLOW PASS PHASE i NETWORK MODULATOR FILTER 2 MODULATOR L!- 2Ti'TRANsM'TTER s i i 1' I SQUARING K22 DRIVER I CIRCUIT 36 AMPLlFlER lLOWPASS H INVERSION DIGJTAL I FILTER I: FREQUENCY c005 g, 1001' IOSCILLATOR GENERATOR 3E .J

1 SQUARING I I CIRCUIT I r r c DE I i OSCILLATOR do l VOICE PRIVACYDEVICE BACKGROUND OF THE INVENTION 1970 and assigned to the assignee ofthe present invention.

2. Prior Art A voice privacy coder system scrambles voice communicationsprior to their transmission over an exposed transmission link. Thisscrambling may be accomplished in many different ways.

In one form of scrambling heretofore proposed, the communication whichis to be privately transmitted is modulated at a number of differentfrequencies which are harmonically related to one another, and one ofthe sidebands at each of these frequencies is then transmitted. Thissystem, while simple and easy to implement, achieves only a moderatedegree of privacy since the fundamental frequency and its harmonics arereadily detected and the transmitted signal therefore readilyunscrambled.

Another system which has been proposed quantizes the analog signal to betransmitted so as to form a binary coded pulse train which representsthe magnitude of the analog signal at various points of time. Certain ofthe digits in this pulse train are then set equal to their complementaryvalues (0 or I, as the case may be) in accordance with a selected codewhich is regenerated at the receiver for unscrambling the coded signal.This system provides a high degree of privacy but is rather complex and,therefore, difficult to implement. Further it requires a large bandwidthfor effective reproduction of the communication at the receiver.

A private communication system capable of reproducing voicecommunications in the low audio range (0 to 3 kHz) and having verynarrow bandwidth requirements is described in the copending applicationof Alfred L. Girard et al., Ser. No. 640,665 filed May 23, 1967 andassigned to the same assignee as the present invention. The systemdescribed in that application scrambles a voice communication with acomplex code word. It is suggested in that application that the securityof the communication (i.e., the difficulty with which the communicationmay be recovered from the scrambled signal by unauthorized interceptorsofit) maybe increased by inverting the signal after it has beenscrambled and prior to transmission. Such a system is capable ofproviding greater security with still acceptable fidelity. However,scrambling the voice communication prior to inversion generates a largenumber of modulation products which, if not filtered prior to inversion,may interfere with each other during descrambling, thus degrading tosome extent the quality of the signal.

Other systems have been proposed in which a voice communication to betransmitted in privacy is first single-sideband modulated onto aconstant frequency carrier and is then scrambled with a second signalwhose frequency is varied in accordance with a code word. An example ofone such system is described in an article by LE. Zegers appearing inVol. COM-16, No. 6 of the IEE Transactions on Communication Theory,Dec., 1968. Such a system requires a very stable local oscillator in thereceiver to generate the demodulating signal for the single sidebandtransmission and, additionally, requires a highly stable very linear,controlled oscillator in the receiver to generate the appropriatedescrambling signal in accordance with the locally-regenerated codeword. These oscillators greatly increase the cost of such a system.Further, because phase synchronization cannot be maintained, the qualityof the decoded signal is degraded.

SUMMARY OF THE INVENTION A; Objects of the Invention Accordingly, it isan object of the invention to provide an improved voice privacy codersystem.

Further, it is an object of the invention to provide an improved voiceprivacy coder system which has only narrow bandwidth requirements.

Another object of the invention is to provide an improved voice privacycoder system in which the privacy of the communication is enhanced incomparison with other systems without significant additional complexityor degradation of the fidelity and without the need for expensive,highly-stable oscillators.

A further object of the invention is to provide an improved voiceprivacy coder system which is readily adapted to existing non-privatecommunication systems.

Still another object of the invention is to provide an improved voiceprivacy coder system in which a synchronizing signal for reconstructionof the code is buried within the coded communication itself and istransmitted with the coded communication in a common frequency band.

Yet another object of the invention is to provide an improved voiceprivacy coder system in which the synchronizing signal to be transmittedwith the coded communication is itself scrambled prior to itstransmission.

B. Brief Description of the Invention In accordance with the foregoingobjects, the voice communication which is to be transmitted in privacyis processed in a coder connected to the voice source. The coder firstinverts the communication, then scrambles it with a complex code word.The inversion is performed by first phase modulating the communicationwith a single pure tone (the inversion tone") and subsequently filteringout the upper sideband modulation product. The resultant signal consistsof a lower sideband only, having the characteristic that the frequencycomponents of this signal are inverted, that is, the lowfrequencycomponents of the voice signal are translated to the high-frequency endof the modulated signal and the high-frequency components of the voicesignal are translated to the low frequency end of the modulated signal.The scrambling is achieved by phase modulating the inverted signaldirectly with the complex code word.

To establish synchronizing signals at the receiver and thereby obviatethe use of highly stable local oscillators, the inversion frequency andthe complex code itself are transmitted with the voice communication.The complex code is transmitted to the receiver directly by adding it ata low level to the scrambled signal prior to transmission, while theinversion frequency tone is preferably added to the inverted voicecommunication prior to its scrambling so that it is scrambled with thecomplex code word along with the inverted voice communication. Thisenhances the privacy of the communication system.

At the receiver, the complex code word is recovered from the receivedsignal by means of a code synchronization loop which performs acorrelation operation on the received signal to regenerate the code. Theregenerated code is then used to descramble the signal. The inversionfrequency tone is recovered from the descrambled signal by means of aphase lock loop which locks a local oscillator to the inversionfrequency. The voice communication is then recovered by re-invertingabout the inversion frequency.

In an alternative embodiment, the complex code word is omitted from thetransmission and the sole synchronizing signal comprises the inversiontone which is added to the inverted voice communication prior toscrambling. The resultant transmitted signal then has two components,namely, the inverted scrambled voice communication and the scrambledinversion tone which serves as the synchronizing signal.

The complex code word and the inversion tone are recovered from thereceived synchronization signal at the receiver by means of a codesynchronization loop and a phase lock loop which are cross coupled toeach other. The input to each loop is through a phase modulator havingthe synchronization signal applied as a first input and having theoutput of the opposite loop as a second input; the output of each loopis then the complex code word and the inversion tone, respectively.

These are used to descram ble the received communication and to thenreinvert it to recover the original voice communication.

DETAILED DESCRIPTION OF THE INVENTION The foregoing and other andfurther objects and features of the invention will be understood moreclearly on reference to the following detailed description of theinvention when taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a schematic diagram of a portion of a radio transmitterconnected to the encoder of the present invention for scrambling acommunication prior to transmission; and

FIG. 2 is a schematic diagram of a portion of a radio receiver connectedto the decoder of the present invention for unscrambling the receivedcommunication.

In FIG. 1, the encoding apparatus of the present invention is interposedbetween an analog signal source 12 (illustratively shown as amicrophone) and a transmitter 14 (which may include radio frequencymodu- After the passage through the network 16, the voice signal isapplied to a phase modulator 18 where it is modulated with a carriersupplied to the modulator from an oscillator 20 through a squaringcircuit 22. The

.oscillator 20 generates a sinusoidal output at a single frequencycalled the inversion frequency; this output is converted by the squaringcircuit to a signal which is substantially a square wave and whichdrives the modulator 18 on and off at precisely defined times. Themodulator 18 may advantageously be formed from a pair of transistorswitches arranged in a balanced configuration and followed by adifference amplifier having its inputs connected to the respectiveswitch outputs. In the time domain, the output of the modulator is ananalog of the audio signal in which successive segments are shifted byin accordance with the applied carrier. In the frequency domain, theoutput of the modulator 18 comprises upper and lower sidebands centeredaround the fundamental frequency of the oscillator 20 and its higherharmonics as contained in the square wave. For voice communications in a300 Hz .3 kHz passband, the inversion frequency is selected to be withinthe passband, e.g. at 2.7 kHz.

The modulator output is applied to a low pass filter 24 which filtersout the upper sideband positioned above the fundamental inversionfrequency, together with all other modulation products centered aroundhigher frequencies. The output of the filter 24 thus comprises a singlelower sideband below the fundamental inversion frequency; this sidebandis hereafter referred to as the inverted voice communication.

The inverted voice communication is added to a portion of the output ofthe oscillator 20 in a summer 26 and this sum is then applied to a phasemodulator 28 having a construction similar to that of the modulator 18.An oscillator 30 drives a digital code generator 32 through a squaringcircuit 34 which converts the sinusoidal output of the oscillator 30 toa uniform square wave; the oscillator may advantageously operate at afrequency of 1 kHz. The code generator 32 in turn drives the balancedmodulator 28 through a driver amplifier 36; it generates a complexdigital code which is used to phase modulate the output of the summer26. Although various forms of digital code generators may be used, thegenerator 32 may advantageously comprise a digital shift register havingone or more feedback connections around portions of the register torepetitively generate a complex code word which is pseudo random innature, that is, the distribution of 1 "s and 0"s in the word is nearlythat of a random distribution. The generator 32 is set to an initialstate by the operator. This state defines a particular code word whichis applied to the modulator-'28 bit by bit at a rate determined by'theoscillator 30. The

word is then continuously repeated during the translating and amplifyingstages). The encoding apparatus 10 includes a wave-shaping network 16for filtering out signal components from the microphone 12 which may lieoutside the bandwidth of the signal to be transmitted. For example, whenspeech is to be transmitted, the network 16 may include filter elementswhich attenuate at frequencies below 300 Hz and above 3 kHz, which isthusthe bandwidth to which the coder must be accommodated.

mission. The inverted voice signal, to which the inversion frequencytone has been added, is scrambled directly with this code word, by meansof the modulator 28, to provide a signal which is unintelligible tothose not having the proper-equipment to reconstitute the voicecommunication. The code word-may be varied by changing the feedbackconnections around the shift register.

The output of the code generator 32 is filtered in a low-pass filter 38and is then added to the scrambled output of the modulator 28 in asummer 40. The filter 38 removes the high frequency components of thecode word prior to transmission to minimize interference with therecovered audio signal on subsequent decoding; the filtered code servesas a second synchronizing signal for recovery of the scrambled voicecommunication as will be seen below. To make detection more difficult,the sigial level of the code should preferably be of the order ofpercent of the signal level of the scrambled communication.

The output of the summer 40 thus comprises a scrambled inverted versionof the voice communication to which the complex code has been added at alow level. This composite signal is applied to the transmitter 14 fortransmission to a remote receiver. The transmitter 14 performsappropriate wave-shaping and frequency fianslating operations on thescrambled signal prior to its transmission to the remote receiver. Forexample, when the encoding apparatus is used in a mobile two-way radiocommunication system, the scrambled signal will be radio-frequencymodulated onto a carrier signal whose frequency is of the order of 100mliz or more. In such a case, the scrambled signal will occupy only avery small bandwidth centered at the frequency of the carrier. When thetransmission link is to comprise a telephone transmission line, on theother hand, the transmitter 14 need not necessarily contain frequencytranslating circuitry; indeed the transmitter may be eliminatedaltogether. The reason for enclosing the filter 38 and summer 40 in arectangular box and for showing a dotted line bypass around the summer40 and between the modulator 28 and the transmitter 14 will be made moreclear below in connection with a discussion of an alternative embodimentof the transmitter of FIG. 1. For the present, these modifications maybe ignored.

In FIG. 2, decoding apparatus 50 is shown embodied in a radio receiver.The receiver incorporates a radio frequency demodulator and amplifier 52for recovering the scrambled communication from the carrier frequencyonto which it has been superimposed. The scrambled communication is thenapplied to a phase modulator 54, a low-pass filter and limiter 56, and aphase modulator 58. The presence of a phase modulator 59 may be ignoredfor the moment.

The filter and limiter 56 has a 3 db cutofl at approximately thefrequency of the code word so that the signal components at higherfrequencies are rejected by it. The output of the filter 56 is appliedto a code synchronization loop 60 comprising a digital correlator 62, alow-pass filter 64 which may include an active amplifying element, avoltage controlled oscillator 66, a squaring circuit 68 and a codegenerator 70. The correlator 62 receives inputs from the filter 56 andthe generator 70. The generator 70 in turn is driven from the oscillator66. through the squaring circuit 68. The output of the correlator 62 isapplied through the filter 64 as a control input to the oscillator 66.

The code generator 70 may advantageously comprise a digital shiftregister similar to the register 28 in the transmitter and initially setto the same code word. The contents of the register are shifted bit bybit in response to driving signals from the oscillator 66 until thecorresponding bits in the transmitter code generator and receiver codegenerator are aligned or synchronized. The frequency of the oscillator66 is controlled by the output of the correlator 62 which, in turn, isdependent on the output of both the code generator 70 and the filter andlimiter 56. The correlator 62 may simply comprise a pair of AND gatesdriving the positive and negative inputs, respectively, of a dif ferenceamplifier connected in an integrating configuration.

The code synchronization loop 60 extracts the low level complex codefrom the scrambled signal for subsequent use in descrambling the signal.It does this by correlating the received signal with the locallygenerated complex code word and then advancing or delaying the shiftingof bits in the shift register of generator 70 until the generated codeis synchronized with the received code. When the code generated by thegenerator 70 is in phase with the code contained in the scrambledsignal, the gates in the correlator altern'ately drive the positive andnegative inputs of the integrating amplifier; these driving signals areintegrated to provide a zero net contribution to the amplifier outputover the integrating period. As long as the output of the code generator70 is in phase with the code contained in the scrambled signal,therefore, the amplifier output remains constant and the oscillator 66supplies a constant frequency output.

If now, the output of the generator 70 is out of phase with the codeword in the scrambled signal, one of the correlator gates will be openedfor a greater length of time that the other of the gates, the particulargate being dependent on the direction of the phase displacement betweenthe code word in the scrambled signal and the code generated by thegenerator 70. The uneven opening of the gates drives the integratingamplifier in one direction or the other to provide a net (non-zero)output which causes the voltage controlled oscillator 66 to change itsinstantaneous frequency in one direction or the other in accordance withthe phase displacement between the two signals; this drives thegenerator 70 in a direction such as to minimize this phase displacement.Thus, the generator 70 is locked onto the code word in the scrambledsignal to reproduce the code word at the receiver.

To obtain a shorter synchronization time for the complex code word, itis desirable to operate the oscillator 66 at a slightly higher rate thanthe bit rate of the code word itself during the acquisition cycle andthen switch to the synchronous rate once the code word has beenacquired. This frequency offset is readily obtained by introducing abias voltage into the active amplifying element of filter 64 during thetime the code word is being acquired so as to operate the oscillator ata frequency above the synchronous frequency and then removing this biasafter synchronization has been achieved. The bias switching isadvantageously performed in-response to a switching signal applied tothe amplifying element over a lead 64 from an acquisition detectioncircuit described in more detail below. Thus, prior to synchronization,the bias offset of this element is set such as to drive the oscillator66 to a frequency of the order of 50-100 l-Iz above the bit rate of thecode word. When synchronization is achieved, the offset is removed so asto drive the oscillator to a frequency corresponding to the synchronousfrequency, that is, a frequency equal to the bit rate of the code word.

Simultaneous with the switching of the bias voltage for the frequencyoffset, the noise bandwidth of the loop should be changed from a highvalue during acquisition of the code to a lower value aftersynchronization has been obtained. The increased bandwidth duringacquisition assists in ensuring rapid synchronization but would degradesignal quality after synchronization if not reduced at that time. Thechange in bandwidth is readily accomplished by changing the gain of theactive amplifying element after synchronization is achieved and ispreferably done in response to the same switching signal as is utilizedfor switching the bias voltage for the frequency offset.

The output of the generator 70 is applied tothe modulator 58 and also toa switch 72 associated with the modulator S4. The scrambled, invertedcommunication at the output of the modulator 52 is unscrambled in themodulator 58 by remodulating it with the complex code word regeneratedin the generator 70 of code synchronization loop 60. The descrambledoutput, which comprises the inversion tone plus the inverted voicecommunication is applied to a band pass filter and limiter 74 having acenter frequency at the inversion frequency.

The filter 74 passes only the pure tone at the inversion frequency andthis is applied to a phase lock loop 76 comprising a phase comparator78, an integrator 80, a voltage controlled oscillator 82, and a phaseshifter and limiter 84 connected in a closed loop. The comparator 78compares the phase of the recovered inversion tone with respect to thephase of the oscillator 82.

the switches 72 and 88 and passes the code word and the inversionfrequency tone through to the modulators 54 and 86, respectively. Italso supplies a signal to the amplifier stage in the filter andamplifier 64 to decrease the gain of the amplifier and thus switch theoscillator 66 to the synchronous frequency. The modulator 54 descramblesthe communication and supplies as an output a signal comprising theinverted voice communication plus the inversion frequency tone. Thissignal is reinverted by the modulator 86 and then passed through a lowpass filter 94 to an output utilization device, such as a speaker 96.The modulator 86, as well as the modulators 54 and 58, mayadvantageously have If the two signals are out of phase, the comparator78 provides an output which, when integratedby the integrator 80,provides a driving signal to the oscillator 82 to shift its frequency inone direction or the other to minimize the phase difference. The phaseshifter and limiter 84 provides a 90 phase shift to the oscillatoroutput for proper operation of the comparator 78 and also transforms thesinusoidal output of the oscillator into a square wave so that thecomparator is switched on and off at precisely defined intervals. Thus,the oscillator 82 is made to generate an output which is in' phase withthe inversion tone contained in the received signal. At this point, boththe complex code and the inversion tone have been regenerated, withproper phase, at the local receiver so that the voice communication maynow be recovered from the scrambled, inverted signals.

the same construction as the modulators 18 and 28 in the transmitter.

The switches 72 and 88, in addition to preventing a garbled output priorto the time that the complex code and the inversion tone have beenregenerated at the receiver, serve the additional function of permittingthrough-transmission of uncoded signals. Thus, when each of themodulators 54 and 86 comprises a pair of transistors arranged insymmetric, balanced, configuration, the transistors function as ordinaryamplifiers in the absence of a modulating signal applied to them.Accordingly, in the absence of a complex code component in the receivedsignal, the modulators 54 and 86 merely amplify the received signal andpass it through to the output unchanged in other respects. When such acomponent is present in the received signal, they recover the originalcommunication by successive remodulation. Thus, whether the receivedsignal is scrambled or unscrambled, the output will always be a clearreproduction of the voice communication.

Instead of transmitting the complex word with the inverted, scrambledcommunication to thereby obtain a synchronization signal, it is possibleto utilize the scrambled inversion tone itself as the solesynchronization signal and to reconstitute the inversion tone and thecode receiver by cross-coupling the code synchronization loop and thephase-lock loop such that the output of the one serves as the input ofthe other.

The descrambling and reinversion of the received modulators, the outputof the decoder consists of unintelligible, garbled signals. To preventtransmission of these signals to the output prior to decoding, themodulators 54 and 86 are keyed on by switches 72 and 88, respectively.

The switch 72, when on, passes the regenerated code word from thegenerator to the modulator 54, while the switch 88, when on, passes theinversion tone from the oscillator 82 to the modulator 86. Theseswitches in turn are operated from a switch through a low pass filter92. The switch 90 is driven from oscillator 82. When the output of theoscillator 82 and the limiter 74 are in phase, the switch 90 is closed;this in turn closes The modifications required to effectuate this areshown in dotted lines in FIGS. 1 and 2. In FIG. 1, the low pass filter38 and summer 40 enclosed within the block are omitted in thealternative embodiment and instead the output of the phase modulator 28is connected directly to the transmitter 14 via the lead 102. Thus, thecode from the generator 32 is applied only to the amplifier 36 and isnot applied to the transmitter 14. Since the signal input to themodulator 28 is the sum of the inverted voice communication and theinversion tone, the output of this modulator is the inverted, scrambledvoice communication plus the scrambled inversion tone which serves as asynchronizing signal for recovering the original communication at thereceiver. Preferably, the synchronization component is at a level ofapproximately 25 percent of that of the communication component. 7

Referring now to FIG. 2, the received signal, consisting of acommunication component (the inverted, scrambled voice) and asynchronization component (the scrambled inversion tone) is firstdemodulated by the demodulator 52 (assuming the signal to have beenmodulated by the transmitter 14 prior to its transmission) and thenapplied to the phase modulators 54 and 58, previously described, as wellas to an additional phase modulator 59 required in the alternativeembodiment now being described. The modulator 59 is driven by the outputof the phase lock loop 76 and supplies its output to the filter 56. Thelead 56' which formerly connected the filter 56 to the demodulator 52 isremoved in the present embodiment so that the sole input to the filter56 is from the modulator 59.

The communication and synchronization components of the demodulatedinput signal are mixed with V the complex code word in the demodulator58 and with the inversion tone in the demodulator 59. Assume for themoment that the code synchronization loop 60 is locked to the codeportion of the incoming synchronization signal so that the generator 70drives the demodulator 58 with the regenerated complex code word. Thisis mixed with the communication and synchronization signal componentsapplied to the demodulator to provide a first output comprising theinverted voice communication and a secondoutput comprising the inversiontone, together with higher order modulation products. The inverted voicecommunication and the higher order modulation products are filtered outby the filter 74 so that only the pure inversion tone passes through thefilter to drive the phase lock loop 76. The inverted scrambledcommunication component, and all higher order components arising fromdemodulation of the synchronization component, are filtered out by thefilter 56 so that the input to the synchronization loop consistsprimarily of the complex code. The synchronization loop 60 thenregenerates this code sequence free of noise and distortion. Thisregenerated code, together with the regenerated inversion tone, isapplied to the demodulators 54 and 86 respectively, which firstdescramble the communica tion component and then re-invert it in themanner previously described. The resultant signal is then passed throughthe low pass filter 94 for reproduction by the speaker 96.

The alternative embodiment described above posseses certain advantagesover the embodiment first described herein. Thus, the voice privacydevice may readily be incorporated in a receiver which has a poorresponse in the low frequency ranges (-300 Hz), since thesynchronization component is entirely within the frequency band abovethis lower limit and thus is within the pass band of the receiver.Further, the system is more tolerant of frequency offset errors such asmay occur in single sideband radio transmission or in transmission overordinary telephone lines.

Finally, when the received signal is passed through the voicedemodulators, the demodulation components due to the synchronizationsignal fall largely outside the voice passband and thus are nottransmitted through the system with the descrambled speech; this resultsin a decreased background noise level.

From the foregoing, it will be seen that l have provided an improvedvoice privacy coder system which achieves a relatively high level ofprivacy while maintaining good fidelity of reproduction. This is 'due inpart, in both embodiments to inversion of the voicecommunication priorto scrambling with a complex code; this pre-inversion increases thescrambling 6 A synchronization signal within the frequency band of thetransmitted voice communication and comprising, in one embodiment, thescrambled inversion tone plus the complex code word or, in the otherembodiment, the scrambled inversion tone itself, is sent along with thetransmitted communication and is separated into its component parts atthe receiver so that the voice communication may be recovered. Embeddingthe synchronization signal within the communication signal in thismanner further increases voice privacy.

The voice privacy coder systems of the present invention have been shownas embodied in a wireless transmitter-receiver combination. However, theinvention is not so limited and the coders may advantageously be appliedto other purposes, for example, to encoding and decoding communicationswhich are to be transmitted over telephone lines. Further, any analogsignal may be sent in privacy with the present invention, whether or notarising from a voice source. Other applications will suggest themselvesto those skilled in the art.

Having described a preferred embodiment of the invention,

I claim:

1. Apparatus for encoding an analog communication in the audio frequencyrange for transmission to a remote receiver through a system having abandwidth limited to the bandwidth of the analog communication,comprising:

A. means for inverting the analog-communication around a frequencywithin the bandwidth of the communication;

B. means for adding a signal corresponding to the inversion frequency tothe inverted communication to form a composite signal to be scrambled;

C. a code generator for repetetively generating a pseudo-random digitalcode word;

D. a phase modulator for scrambling said composite signal directly withsaid pseudo-random digital code word at a rate within the bandwidth ofthe communication; and

E. means for adding to the scrambled signal, prior to the transmissionthereof, a synchronizing signal indicative of the phase of the complexcode word, whereby said code may be regenerated at the remote receiverin synchronism with the modulating code at the transmitter.

2. Apparatus for decoding an analog communication which has beeninverted and then scrambled directly with a complex code word prior totransmission to the decoding apparatus, and which includes synchronizingsignal components corresponding to the inversion tone and the complexcode word, said apparatus comprising:

A. means for generating a replica of the complex code word insynchronism with the code word used to scramble said communication;

8. means for generating a tone at the inversion frequency in synchronismwith the tone with which the communication has been scrambled;

C. means for descrambling the coded communication with the generatedcode word;

D. means for re-inverting the unscrambled communication with thegenerated tone; and

E. means for preventing the descrambling or the reinverting of the codedcommunication prior to the generation of the code word replica and theinversion frequency tone at the receiver.

3. Apparatus for decoding an analog communication which has beeninverted and then scrambled by phasemodulating it directly with acomplex code word prior to transmission to the decoding apparatus, andwhich includes synchronizing signal components corresponding to theinversion tone and the complex code word, said apparatus comprising:

A. means for generating a replica of the complex code word insynchronism with the code word used to scramble said communication;

B. means for generating a tone at the inversion frequency in synchronismwith the tone with which the communication has been scrambled;

C. means for descrambling the coded communication by phase-demodulatingit with the generated code word in a balanced modulator having switchingmeans associated therewith for inhibiting the application of the codereplica to the modulator prior to the time that the replica has beensynchronized with the code word component of the received codedcommunication, whereby the received communication is passed through saidmodulator without frequency translation in the absence ofsynchronization; and

D. means for re-inverting the unscrambled communication with thegenerated tone.

4. Apparatus for decoding an analog communication which has beeninverted and then scrambled directly with a complex code word prior totransmission to the decoding apparatus, and which includes synchronizingsignal components corresponding to the inversion tone and the complexcode word, said apparatus comprising:

A. means for generating a replica of the complex code word insynchronism with the code word used to scramble said communication;

B. means for generating a tone at the inversion frequency in synchronismwith the tone with which the communication has been scrambled, saidmeans comprising:

I. an oscillator supplying an output at a controlla-v ble rate,

2. a phase comparator i. connected to the oscillator,

ii. receiving a first input from the oscillatorand a.

second input from the inversion tone component in the received codedsignal,

iii. supplying a first driving signal to the oscillator to shift itsfrequency in a direction such as to minimize the phase differencebetween the oscillator output and the inversion tone component in thereceived coded communication when the oscillator output and theinversion tone component are not synchronized with each other, and

iv. supplying a second driving signal-to the oscillator to maintain theoscillator frequency constant when the oscillator output and theinversion tone component are synchronized with each other; C. means fordescrambling the coded communication with the generated code word; andD. means for re-inverting the unscrambled communication with thegenerated tone. 5. Apparatus according to claim 4 in which the inversiontone component of the received coded communication is scrambled with theinverted communication prior to transmission and is recovered from saidcommunication by rescrambling said communication with the replica of thecomplex code word prior to applying said tone component to said phasecomparator.

6. Apparatus for decoding an analog communication which has beeninverted and then scrambled directly with a complex code word prior totransmission to the decoding apparatus, and which includes synchronizingsignal components corresponding to the inversion tone and the complexcode word, said apparatus comprising:

A. means for generating a replica of the complex code word insynchronism with the code 7 word used to scramble said communication,said means comprising a code synchronizing loop having:

l. a feedback shift register for generating a digital pattern, a digitat a time,

2. an oscillator connected to said register to supply shift signalsthereto at a controllable rate; and 3. a correlator i. receiving a firstinput from the coded communication and a second input from the shiftregister,

ii. supplying a first driving signal to the oscillator to shift itsfrequency in a direction such as to minimize the phase differencebetween the oscillator output and the code word component in thereceived coded communication, and

iii. supplying a second driving signal for maintaining the oscillatorfrequency constant when the output of the shift register is synchronizedwith said component;

B. means for generating a tone at the inversion frequency in synchronismwith the tone with which the communication has been scrambled;

C. means for descrambling the coded communication with the generatedcode word; and

D. means for re-inverting the unscrambled communication with thegenerated tone.

7. Apparatus according to claim 6 including:

A. means connected to the shift register for dcscrambling the codedcommunication with the code word replica prior to generation of theinversion tone, and

B. switch means 1. responsive to the phase relation between the locallygenerated inversion tone and the inversion tone with which thecommunication has been .encoded,and

2. providing a switching command indicative of synchronization betweenthe respective inversion tones.

8. A voice privacy communication system for the transmission of analogcommunications in privacy through a transmission channel having limitedbandwidth, comprising:

A. a transmitter having 1. means for inverting an analog communicationin accordance with an inverting signal;

2. means for adding a signal corresponding to the inverting signal tothe inverted communication to form a composite signal for scrambling;

3. means for scrambling the composite signal directly with a complexcode word, and

4. means for including synchronizing signals within the encodedcommunication prior to transmission to a remote receiver; and

B. a receiver having,

1. means for generating a replica of the complex code word insynchronism with the code word with which the communication has beenscrambled,

2. means for unscrambling the received encoded communication with thecomplex code word replica,

3. means for generating a replica of the inverting signal in synchronismwith the signal inverting the analog communication, and

. means for reinverting the unscrambled communication in accordance withthe inverting signal replica to recover the analog communication.

9. A communication system according to claim 8 in which the complex codeword is added to the coded communication at a power level substantiallyless than the power level of the coded communication prior totransmission of said communication to the remote receiver.

10. A voice privacy communication system for the transmission of analogcommunications in privacy through a transmission channel having limitedbandwidth, comprising:

A. a transmitter having 1. means for inverting an analog communicationin accordance with an inverting signal,

2. means for scrambling the inverted communication directly with acomplex code word, and i 3. means for including synchronizing signalswithin the encoded communication prior to transmission to a remotereceiver; and t B. a receiver having 1. means for generating a replicaof the complex code word in synchronism with the code word with whichthe communication has been scrambled, said means comprising a codesynchronizing loop having:

i. a digital shift register in which the code word replica is stored andsupplying outputs from at least two distinct portions of said word,

ii. an oscillator applying shift signals to the register to shift theword position therein, and

iii. means for comparing the phase of the replica with the phase of thecode word component contained in the received communication, thecomparing means providing a first output to control the frequency withwhich shift signals are applied to the shift register when the code wordand its replica are not synchronized and providing a second output tofix the frequency with which the shift signals are applied to the shiftregister when the code word and its replica are synchronized;

2. .means for unscrambling the received encoded communication with thecomplex code word replica,

3. means for generating a replica of the inverting signal in synchronismwith the signal inverting the analog communication, and

. means for reinverting the unscrambled communication in accordance withthe inverting signal replica to recover the analog communication.

11. A communication system according to claim 10 in which the means forgenerating the replica of the inverting signal includes:

A. a controllable oscillator providing an output, and

B. a phase comparator 1. receiving the oscillator output as a firstinput and the unscrambled communication as a second input,

2. providing a first driving signal to the oscillator when the first andsecond inputs to the comparator are not synchronized in phase to therebychange the frequency of the oscillator output, and

3. providing a second driving signal to the oscilla tor when the firstand second inputs to the comparator are synchronized in phase to therebymaintain the oscillator frequency constant.

12. A communication system according to claim 11 which includesswitching means responsive to the oscillator output to prevent frequencytranslation of the unscrambled communication prior to the time that thecode word replica and the inverting signal replica are generated insynchronism with their respective counterparts.

13. A communication system according to claim 10 which includes meansfor setting the oscillator to a first frequency prior to the time thecomplex code word is generated and for setting the oscillator to asecond frequency after the code word is generated.

14. A communication system according to claim 13 in which the frequencysetting means comprises means for supplying an offset bias in the codesynchronizing loop in accordance with the presence or absence of asynchronizing signal indicating the achievement of synchronization.

15. Apparatus according to claim 13 including means for varying the loopgain in the code synchronizing loop in accordance with the presence orabsence of a code synchronizing signal indicating the achievement ofsynchronization to thereby vary the noise bandwidth of the loop.

16. Apparatus for encoding an analog communication for transmission to aremote receiver, comprising:

A. means for inverting the analog communication around a selectedfrequency;

B. means for adding to the inverted communication a signal correspondingto the inversion frequency to form a composite signal to be scrambled.

C. a code generator for repetitively generating a pseudo-random digitalcode word; and

D. means for scrambling the composite signal directly with said codeword to thereby provide synchronization and communication components,respectively for simultaneous transmission to a remote receiver over acommon channel.

17. Apparatus according to claim 16 in which the means for inverting thecommunication comprises:

A. a modulator having the communication applied as a first input theretoand a tone at the inversion frequency as a second input thereto; and

B. a filter for eliminating the upper sideband from the modulatedcommunication.

18. Apparatus according to claim 16 in which the means for scramblingthe composite signal comprises a modulator having the composite signalapplied as a first input thereto and the output of the code generatorapplied as a second input thereto.

synchronization signal transmitted to the remote receiver.

20. Apparatus according to claim 16 in which the inversion tone is addedto the inverted communication prior to scrambling whereby the invertedcommunication and the inversion tone are both scrambled with the complexcode simultaneously.

21. Apparatus according to claim 16 in which the means for scramblingthe composite signal comprises a phase modulator having the compositesignal applied as a first input thereto and the code word applied as asecond input thereto.

22. Apparatus for decoding a scrambled composite signal formed from aninverted communication to which a tone at the inversion frequency hasbeen added prior to scrambling, said apparatus comprising A. a codesynchronization loop providing as an output a regenerated version of thecomplex code at the remote receiver;

B. a phase lock loop providing as an output a regenerated version of theinversion tone in phase with the inversion tone carried in thesynchronizing signal;

C. a first descrambler (i) having the scrambled composite signal appliedas a first input thereto, (ii) having the output of the phase lock loopconnected as a second input thereto, and (iii) supplying to the codesynchronization loop an input comprising a reinverted version of thescrambled communication.

23. Apparatus according to claim 22 which includes a second descrambler(i) having the scrambled composite signal applied as a first inputthereto, (ii) having the output of the code synchronization loop appliedas a second input thereto, and (iii) supplying to the phase lock loop aninput comprising a descram bled version of the inversion tone.

24. Apparatus according to claim 22 in which said inversion tone andsaid communication are scrambled by modulating them with said complexcode word and in which said first and second descramblers respectivelycomprise modulators.

25. Apparatus according to claim 1 in which the modulators are phasemodulators.

26. Apparatus according to claim 25 which includes first and secondfilters associated with the first and second modulators transmittingtherethrough substantially only the code and inversion tonerespectively, the remaining modulation components being blocked by saidfilters.

1. Apparatus for encoding an analog communication in the audio frequencyrange for transmission to a remote receiver through a system having abandwidth limited to the bandwidth of the analog communication,comprising: A. means for inverting the analog communication around afrequency within the bandwidth of the communication; B. means for addinga signal corresponding to the inversion frequency to the invertedcommunication to form a composite signal to be scrambled; C. a codegenerator for repetetively generating a pseudo-random digital code word;D. a phase modulator for scrambling said composite signal directly withsaid pseudo-random digital code word at a rate within the bandwidth ofthe communication; and E. means for adding to the scrambled signal,prior to the transmission thereof, a synchronizing signal indicative ofthe phase of the complex code word, whereby said code may be regeneratedat the remote receiver in synchronism with the modulating code at thetransmitter.
 2. providing a first driving signal to the oscillator whenthe first and second inputs to the comparator are not synchronized inphase to thereby change the frequency of the oscillator output, and 2.Apparatus for decoding an analog communication which has been invertedand then scrambled directly with a complex code word prior totransmission to the decoding apparatus, and which includes synchronizingsignal components corresponding to the inversion tone and the complexcode word, said apparatus comprising: A. means for generating a replicaof the complex code word in synchronism with the code word used toscramble said communication; B. means for generating a tone at theinversion frequency in synchronism with the tone with which thecommunication has been scrambled; C. means for descrambling the codedcommunication with the generated code word; D. means for re-invertingthe unscrambled communication with the generated tone; and E. means forpreventing the descrambling or the reinverting of the codedcommunication prior to the generation of the code word replica and theinversion frequency tone at the receiver.
 2. means for unscrambling thereceived encoded communication with the complex code word replica, 2.means for scrambling the inverted communication directly with a complexcode word, and
 2. means for unscrambling the received encodedcommunication with the complex code word replica,
 2. means for adding asignal corresponding to the inverting signal to the invertedcommunication to form a composite signal for scrambling;
 2. providing aswitching command indicative of synchronization between the respectiveinversion tones.
 2. an oscillator connected to said register to supplyshift signals thereto at a controllable rate; and
 2. a phase comparatori. connected to the oscillator, ii. receiving a first input from theoscillator and a second input from the inversion tone component in thereceived coded signal, iii. supplying a first driving signal to theoscillator to shift its frequency in a direction such as to minimize thephase difference between the oscillator output and the inversion tonecomponent in the received coded communication when the oscillator outputand the inversion tone component are not synchronized with each other,and iv. supplying a second driving signal to the oscillator to maintainthe oscillator frequency constant when the oscillator output and theinversion tone component are synchronized with each other; C. means fordescrambling the coded communication with the generated code word; andD. means for re-inverting the unscrambled communication with thegenerated tone.
 3. means for generating a replica of the invertingsignal in synchronism with the signal inverting the analogcommunication, and
 3. a correlator i. receiving a first input from thecoded communication and a second input from the shift register, ii.supplying a first driving signal to the oscillator to shift itsfrequency in a direction such as to minimize the phase differencebetween the oscillator output and the code word component in thereceived coded communication, and iii. supplying a second driving signalfor maintaining the oscillator frequency constant when the output of theshift register is synchronized with said component; B. means forgenerating a tone at the inversion frequency in synchronism with thetone with which the communication has been scrambled; C. means fordescrambling the coded communication with the generated code word; andD. means for re-inverting the unscrambled communication with thegenerated tone.
 3. means for scrambling the composite signal directlywith a complex code word, and
 3. Apparatus for decoding an analogcommunication which has been inverted and then scrambled byphase-modulating it directly with a complex code word prior totransmission to the decoding apparatus, and which includes synchronizingsignal components corresponding to the inversion tone and the complexcode word, said apparatus comprising: A. means for generating a replicaof the complex code word in synchronism with the code word used toscramble said communication; B. means for generating a tone at theinversion frequency in synchronism with the tone with which thecommunication has been scrambled; C. means for descrambling the codedcommunication by phase-demodulating it with the generated code word in abalanced modulator having switching means associated therewith forinhibiting the application of the code replica to the modulator prior tothe time that the replica has been synchronized with the code wordcomponent of the received coded communication, whereby the receivedcommunication is passed through said modulator without frequencytranslation in the absence of synchronization; and D. means forre-inverting the unscrambled communication with the generated tone. 3.means for including synchronizing signals within the encodedcommunication prior to transmission to a remote receiver; and B. areceiver having
 3. means for generating a replica of the invertingsignal in synchronism with the signal inverting the analogcommunication, and
 3. providing a second driving signal to theoscillator when the first and second inputs to the comparator aresynchronized in phase to thereby maintain the oscillator frequencyconstant.
 4. means for reinverting the unscrambled communication inaccordance with the inverting signal replica to recover the analogcommunication.
 4. means for including synchronizing signals within theencoded communication prior to transmission to a remote receiver; and B.a receiver having
 4. means for reinverting the unscrambled communicationin accordance with the inverting signal replica to recover the analogcommunication.
 4. Apparatus for decoding an analog communication whichhas been inverted and then scrambled directly with a complex code wordprior to transmission to the decoding apparatus, and which includessynchrOnizing signal components corresponding to the inversion tone andthe complex code word, said apparatus comprising: A. means forgenerating a replica of the complex code word in synchronism with thecode word used to scramble said communication; B. means for generating atone at the inversion frequency in synchronism with the tone with whichthe communication has been scrambled, said means comprising: 5.Apparatus according to claim 4 in which the inversion tone component ofthe received coded communication is scrambled with the invertedcommunication prior to transmission and is recovered from saidcommunication by rescrambling said communication with the replica of thecomplex code word prior to applying said tone component to said phasecomparator.
 6. Apparatus for decoding an analog communication which hasbeen inverted and then scrambled directly with a complex code word priorto transmission to the decoding apparatus, and which includessynchronizing signal components corresponding to the inversion tone andthe complex code word, said apparatus comprising: A. means forgenerating a replica of the complex code word in synchronism with thecode word used to scramble said communication, said means comprising acode synchronizing loop having:
 7. Apparatus according to claim 6including: A. means connected to the shift register for descrambling thecoded communication with the code word replica prior to generation ofthe inversion tone, and B. switch means
 8. A voice privacy communicationsystem for the transmission of analog communications in privacy througha transmission channel having limited bandwidth, comprising: A. atransmitter having
 9. A communication system according to claim 8 inwhich the complex code word is added to the coded communication at apower level substantially less than the power level of the codedcommunication prior to transmission of said communication to the remotereceiver.
 10. A voice privacy communication system for the transmissionof analog communications in privacy through a transmission channelhaving limited bandwidth, comprising: A. a transmitter having
 11. Acommunication system according to claim 10 in which the means forgenerating the replica of the inverting signal includes: A. acontrollable oscillator providing an output, and B. a phase comparator12. A communication system according to claim 11 which includesswitching means responsive to the oscillator output to prevent frequencytranslation of the unscrambled communication prior to the time that thecode word replica and the inverting signal replica are generated insynchronism with their respective counterparts.
 13. A communicationsystem according to claim 10 which includes means for setting theoscillator to a first frequency prior to the time the complex code wordis generated and for setting the oscillator to a second frequency afterthe code word is generated.
 14. A communication system according toclaim 13 in which the frequency setting means comprises means forsupplying an offset bias in the code synchronizing loop in accordancewith the presence or absence of a synchronizing signal indicating theachievement of synchronization.
 15. Apparatus according to claim 13including means for varying the loop gain in the code synchronizing loopin accordance with the presence or absence of a code synchronizingsignal indicating the achievement of synchronization to thereby vary thenoise bandwidth of the loop.
 16. Apparatus for encoding an analogcommunication for transmission to a remote receiver, comprising: A.means for inverting the analog communication around a selectedfrequency; B. means for adding to the inverted communication a signalcorresponding to the inversion frequency to form a composite signal tobe scrambled. C. a code generator for repetitively generating apseudo-random digital code word; and D. means for scrambling thecomposite signal directly with said code word to thereby providesynchronization and communication components, respectively forsimultaneous transmission to a remote receiver over a common channel.17. Apparatus according to claim 16 in which the means for inverting thecommunication comprises: A. a modulator having the communication appliedas a first input thereto and a tone at the inversion frequency as asecond input thereto; and B. a filter for eliminating the upper sidebandfrom the modulated communication.
 18. Apparatus according to claim 16 inwhich the means for scrambling the composite signal comprises amodulator having the composite signal applied as a first input theretoand the output of the code generator applied as a second input thereto.19. Apparatus according to claim 16 in which the scrambled inversiontone comprises the sole synchronization signal transmitted to the remotereceiver.
 20. Apparatus according to claim 16 in which the inversiontone is added to the inverted communication prior to scrambling wherebythe inverted communication and the inversion tone are both scrambledwith the complex code simultaneously.
 21. Apparatus according to claim16 in which the means for scrambling the composite signal comprises aphase modulator having the composite signal applied as a first inputthereto and the code word applied as a second input thereto. 22.Apparatus for decoding a scrambled composite signal formed from aninverted communication to which a tone at the inversion frequency hasbeen added prior to scrambling, said apparatus comprising A. a codesynchronization loop providing as an output a regenerated version of thecomplex code at the remote receiver; B. a phase lock loop providing asan output a regenerated version of the inversion tone in phase with theinversion tone carried in the synchronizing signal; C. a firstdescrambler (i) having the scrambled composite signal applied as a firstinput thereto, (ii) having the output of the phase lock loop connectedas a second input thereto, and (iii) supplying to the codesynchronization loop an input comprising a reinverted version of thescrambled communication.
 23. Apparatus according to claim 22 whichincludes a second descrambler (i) having the scrambled composite signalapplied as a first input thereto, (ii) having the output of the codesynchronization loop applied as a second input thereto, And (iii)supplying to the phase lock loop an input comprising a descrambledversion of the inversion tone.
 24. Apparatus according to claim 22 inwhich said inversion tone and said communication are scrambled bymodulating them with said complex code word and in which said first andsecond descramblers respectively comprise modulators.
 25. Apparatusaccording to claim 1 in which the modulators are phase modulators. 26.Apparatus according to claim 25 which includes first and second filtersassociated with the first and second modulators transmittingtherethrough substantially only the code and inversion tonerespectively, the remaining modulation components being blocked by saidfilters.